Chronic lower back pain caused by degenerative disc disease is one of the leading causes of disability in adults. Intervertebral disc degeneration can occur as part of the normal aging process in which the nucleus of the disc dehydrates, reducing the shock absorbing capability of the disc. Patients who fail to obtain adequate pain relief from non-surgical treatment (e.g., rest, pain medication, physical therapy, exercise, epidural steroid injections, chiropractic manipulation, ultrasound, massage, orthotics, etc.) may require spinal surgery to alleviate discogenic pain and disability.
One method of treating degenerative disc disease is spinal fusion or arthrodesis surgery in which the affected vertebrae are fused together using a bone graft. During spinal fusion, a perforated titanium cage may be surgically implanted within the space between two adjacent vertebrae after the pain-generating intervertebral disc is removed. The implanted spinal fusion cage must be appropriately sized to restore the normal disc height at the affected vertebral segment. The fusion cage is packed with bone graft, which grows through the perforated walls of the cage and eventually forms a solid bond or fusion with the adjacent vertebrae to prevent motion in the affected vertebral segment and to reduce chronic discogenic pain.
Currently, surgeons must rely on their experience and “feel” when using intervertebral spreaders or distractors to spread the affected intervertebral segment during total disc replacement (TDR) or fusion surgery. Both the artificial disc and fusion cage must fit in a “snug” intervertebral space that has appropriate ligament tension. Too much distraction by the surgeon performing a TDR procedure will result in the placement of an implant that is too large for the intervertebral segment, reducing the ideal range of motion that the implant can provide. In contrast, insufficient ligament tension will not produce enough force on the endplates of the arthroplasty, resulting in loosening and potential migration of the implant from its optimal position. Similarly, since fusion cages or devices should be placed in a stretched intervertebral segment for better fusion, insufficient ligament tension may result in an unsatisfactory fusion and/or undesired range of motion of the affected vertebral segment.
In addition, anterior and posterior pre- and post-distraction disc heights are important parameters for a surgeon to consider when placing an intervertebral disc arthroplasty. It is believed that there is an optimal window of anterior and posterior disc height that allows the optimal range of motion by the arthroplasty. At present, surgeons must either “eyeball” anterior and posterior disc heights before and after distraction on a fluoroscope, and/or use trial devices to measure the size of the intervertebral space by forcing different test devices into the space between adjacent vertebras until the correct sized one is placed. Such trial or test devices are frequently manufactured from expensive metals, such as titanium, and thus are not easily disposable and, as a result, their use is costly.
Yet another parameter for a surgeon to consider when placing an intervertebral disc arthroplasty is the depth of the patient's vertebral endplate. New arthroplasties should cover the entire vertebral endplate outer-rim because this is where the strongest bone lies. However, neither conventional distractors/spreaders nor fluoroscopes provide a surgeon with the precise depth of the patient's vertebral endplate to select the correct size arthroplasty or fusion device that will cover the maximum amount of outer rim.
It is, therefore, desirable to reduce post-operative complications of spinal fusion and TDR arising from improper selection, sizing and placement of the artificial disc or fusion cage by utilizing a measuring device and method capable of sizing the spinal implant according to the natural dimensions and geometry of the intervertebral disc that is to be replaced or augmented. There is, thus, a need for devices and methods for measuring parameters of an intervertebral disc space in all three dimensions. More particularly, there is a need for devices and methods that enable measurements of the intervertebral disc space to be made in a minimally-invasive manner, are easily disposable and are inexpensive.